A semiconductor device used for a display panel of a liquid crystal display device or an EL (Electroluminescent) display device, for example, a semiconductor portion of a TFT (Thin Film Transistor), is generally formed by using a-Si (amorphous silicon) or poly-Si (polycrystalline silicon).
Si (silicon) does not have a large band gap (for example, single-crystalline Si is 1.1 eV), and absorbs visible light. By irradiation with the light, electrons and holes (carriers) are formed in Si. If a Si film is used for a channel formation region of a TFT, a carrier is generated in the channel formation region by irradiation with the light even in an OFF state. Then, current flows between a source region and a drain region. The current which flows in an OFF state is called “OFF-leak current”. If the current value is high, a display panel does not operate normally. Consequently, a light shielding film is formed so as not to irradiate the Si film with light. However, a process becomes complex when the light shielding film is formed, because a deposition step, a photolithography step, and an etching step are required.
To solve the problem, an attention is paid to a transparent transistor using zinc oxide (ZnO) which is a semiconductor having a larger band gap of 3.4 eV than that of Si. Concerning such a transparent transistor, the band gap is larger than light energy in a visible light band and the visible light is not absorbed. Consequently, it has an advantage that the OFF-leak current does not increase if irradiated with light.
A semiconductor device using ZnO for the channel formation region is disclosed in Reference 1, for example. The structure of the semiconductor device using ZnO is described referring to FIG. 7A.
A semiconductor device in FIG. 7A has a source electrode 1001 and a drain electrode 1002, a ZnO layer 1003 arranged so as to be contacted with the source electrode 1001 and the drain electrode 1002, and a gate insulating layer 1004 stacked over the ZnO layer 1003 and a gate electrode 1005 over an insulating substrate 1000 such as a glass substrate.
For the source electrode 1001 and the drain electrode 1002, a conductive ZnO is used. The conductive ZnO is doped with one of the following: B(boron), Al (aluminum), Ga (gallium), In (indium), or Tl (thallium), which are III group elements; F(fluorine), Cl (chlorine), Br (bromine), or I (iodine), which are VII group elements; Li (lithium), Na (sodium), K (potassium), Rb (rubidium), or Cs (caesium), which are I group elements; and N (nitrogen), P (phosphorus), As (arsenic), Sb (antimony), or Bi (bismuth), which are V group elements.    [Reference 1] Japanese Published Patent Application No. 2000-150900